1. Field of the Invention
The invention relates to an amplifier for low resistance AC voltages sources.
2. Background Information
Amplifier stages for low-resistance AC voltage sources--for example HF receiver input stages in radio receivers--are usually made using bipolar technology with a transistor in a base circuit, or using FET technology with a transistor in a gate circuit. A high linearity can be achieved in the circuit by a high collector current or drain current; for example, with a collector current of 10 mA and a source impedance of 50 ohms, the interception point--this value being the measure for the non-linearities in low-level signal operation--is only 10 dBm. At the same time, the base circuit and drain circuit have very favorable noise properties; for example, amplifier noise factors of 2 dB are obtained with a collector current of 3 mA.
Amplifier stages must frequently--for example in the vicinity of strong transmitters--process signals with high signal levels (+10 dBm or more, corresponding to a peak signal current of 20 mA at 50 ohms input resistance). In order for the amplifier circuit to function linearly, the collector current of the input transistor must be high in relation to this peak signal current; in order to ensure linearity in the circuit, the collector current must be increased accordingly with high input signal levels, and hence the operating point of the circuit moved. However, an increase of this size in the collector current is only possible to a limited extent. For example, doubling the collector current corresponds to an increase in the interception point of only 6 dBm. This is however offset by the fact that the interception point only inadequately characterizes the linearity conditions at high signal levels. Increasing the collector current also has the effect of increasing the noise factor, since the noise sources in the transistor generate a noise power proportional to the collector current; the increased power loss causes problems with the dependability of the circuit.